ABSTRACT The US now requires e-cigarette packaging and advertising to have a single warning about nicotine addiction. FDA has the authority to require additional e-cigarette warnings and has stated that a single e-cigarette warning could ?grow stale over time.? No data are available on whether the current or other e-cigarette warnings affect vaping or undermine the motivation to quit smoking combustible cigarettes. Furthermore, little data are available on how and for whom e-cigarette warnings work, including among vulnerable populations. Ideally, e-cigarette warnings would discourage vaping initiation among non-users, reduce vaping among e- cigarette-only users, and support dual-users eventually quitting use of tobacco products. We propose to study the impact of the current e-cigarette warning and inform development of appropriate new warnings. In Aim 1, we will develop new e-cigarette warnings using data from 3 large surveys. We aim to create warnings that e-cigarette users endorse as being effective and that do not cause unintended consequences among adolescents, young adults, and adults, especially dual users. This evaluation will include the current US e-cigarette warning statement about nicotine addiction. The main product of Aim 1 will be the selection of warnings for use in the Aim 2 RCT. In Aim 2, we will evaluate the impact of e-cigarette warnings on vaping in an RCT. Young adult and adult e-cigarette users will receive 4 new warnings affixed to their e-cigarette devices and refills during the 4-week study, allocated in a random order. We will assess whether the e-cigarette warnings increase e-cigarette quit attempts and examine whether the warnings impact varies for dual users, vulnerable populations (young adult; low-income; and gay, lesbian, and bisexual [GLB] e- cigarette users). We will also examine whether the warnings increase Tobacco Warnings Model (TWM) antecedents to quitting in the context of e-cigarettes. The main product of Aim 2 will be an estimate of the impact of e-cigarette warnings on vaping, and an understanding of which TWM antecedents to behavior explain the impact. Finally, in Aim 3, we will examine whether e-cigarette warnings push e-cigarette users toward smoking. We will examine the impact of e-cigarette warnings on motivation to not smoke (defined as cigarette quit intentions among dual users, and susceptibility to smoke among e-cigarette only users), smoking behavior and inaccurate risk perceptions. We will examine if warnings' impact varies for vulnerable groups and warning size. The final product of Aim 3 will be an estimate of the impact of warnings on unintended consequences, including pushing people toward smoking. Across the three aims, we will develop generalizable principles on how to maximize the impact of e-cigarette warnings without pushing dual users toward smoking. Our findings will create an evidence base that can inform development of effective e-cigarette warnings, warnings and advertising policy in the US and globally, and other nicotine addiction and cancer prevention communication and countermarketing campaigns.